No one actually knows how to lculate what speaker cable they need

I understand completely OP. Just looking generally at this topic.

20 feet long. Must run cables under the house to keep it clean looking per wife. Fireplace in the center. Lyngdorf 2170 and Dali Epicon 6. I make cables and do mods as a part time gig.

The 2170 is not going ever 🙂

Based on your way of thinking, which I don’t agree with totally, I would need thick gauge. I use 7 AWG now.   It comes down to personal preference and listening. However I am always open minded and willing to experiment. You need better conductors than your chosen cable in my opinion. Copper clad aluminum is not as good as I would desire. Thus my post here.

Я использую перемычки для своих динамиков.   кому нужны банановые пробки ......https://www.amazon.com/OxGord-Commercial-Battery-Booster-Starter/dp/B00JK325V0

Your Amp. is class D (Digital PCM). No DF or output impedance is provided:
http://lyngdorf.com/wp-content/uploads/2017/06/tdai-2170-owners-manual_v17.pdf
The 30A output is the only clue but not good enough.
Speakers: conventional coil loaded. Ok.
As so, I'll do an assumption, that the  Lyngdorf 2170 have a DF of 250.
For 20' a 3x0 AWG might be required. For an 8' a 3 AWG would do.

The cables you are using, are too long to my like. I understand your managment desire and other esthetic issues. Fire place steals the mid placment option of the Amp. between the two speakers.

Use what ever cable you think is good for you. The one I used had excelent results. You are welcome to take your favorite.

Mr. sejodiren says: I use jumpers for my speakers. who needs banana plugs...
Enjoy. The contact with the clamps of the jumper cables are at a very little area. A good banana plug is way better on contact. also less risky if it disconnects or the red wire touchs the black...

The link goes to a 4 AWG cable that claims 500A. by the guage table, a 4 AWG can conduct only 60A cont.
or 135A momentary. Just for the record, not that you need that high current.
I also do not like that they are molded in two parallel lines.

Mr. grannyring, you say: 
"Based on your way of thinking, which I don’t agree with totally, "
It is up to you to accept or decline my say.
Please be kind to acknowladge me when and if you get any (other) explanation or way to calculate a speaker cable. 
As far as my search reached, there is nothing else out there.
Most just embrace the usual: high purity cooper (in electricity nothing is made under 99.5%) creyogenic treat, bi wire, burn in, or exotic snake oil :-)
All non sintific says, that never convinced me to solve the relations or get the true answer. So, good luck with that.